Asteroids Caused the Early Inner Solar System Cataclysm

ASTEROIDS CAUSED THE EARLY INNER SOLAR SYSTEM CATACLYSM
From Lori Stiles, University Communications, UA, 520-621-1877
September 15, 2005

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Contact information listed at end of news release
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University of Arizona and Japanese scientists are convinced that
evidence at
last settles decades-long arguments about what objects bombarded the
early
inner solar system in a cataclysm 3.9 billion years ago.

Ancient main belt asteroids identical in size to present-day asteroids
in
the Mars-Jupiter belt -- not comets -- hammered the inner rocky planets
in a
unique catastrophe that lasted for a blink of geologic time, anywhere
from
20 million to 150 million years, they report in the Sept. 16 issue of
Science.

However, the objects that have been battering our inner solar system
after
the so-called Late Heavy Bombardment ended are a distinctly different
population, UA Professor Emeritus Robert Strom and colleagues report in
the
article, "The Origin of Planetary Impactors in the Inner Solar System."

After the Late Heavy Bombardment or Lunar Cataclysm period ended,
mostly
near-Earth asteroids (NEAs) have peppered the terrestrial region.

Strom has been studying the size and distribution of craters across
solar
system surfaces for the past 35 years. He has long suspected that two
different projectile populations have been responsible for cratering
inner
solar system surfaces. But there's been too little data to prove it.

Until now.

Now asteroid surveys conducted by UA's Spacewatch, the Sloan Digital
Sky
Survey, Japan's Subaru telescope and the like have amassed fairly
complete
data on asteroids down to those with diameters of less than a
kilometer.
Suddenly it has become possible to compare the sizes of asteroids with
the
sizes of projectiles that blasted craters into surfaces from Mars
inward to
Mercury.

"When we derived the projectile sizes from the cratering record using
scaling laws, the ancient and more recent projectile sizes matched the
ancient and younger asteroid populations smack on," Strom said. "It's
an
astonishing fit."

"One thing this says is that the present-day size-distribution of
asteroids
in the asteroid belt was established at least as far back as 4 billion
years
ago," UA planetary scientist Renu Malhotra, a co-author of the Science
paper, said. "Another thing it says is that the mechanism that caused
the
Late Heavy Bombardment was a gravitational event that swept objects out
of
the asteroid belt regardless of size."

Malhotra discovered in previous research what this mechanism must have
been. Near the end of their formation, Jupiter and the other outer gas
giant
planets swept up planetary debris farther out in the solar system, the
Kuiper Belt region. In clearing up dust and pieces leftover from outer
solar
system planet formation, Jupiter, especially, lost orbital energy and
moved
inward closer to the sun. That migration greatly enhanced Jupiter's
gravitational influence on the asteroid belt, flinging asteroids
irrespective of size toward the inner solar system.

Evidence that main belt asteroids pummeled the early inner solar system
confirms a previously published cosmochemical analysis by UA planetary
scientist David A. Kring and colleagues.

"The size distribution of impact craters in the ancient highlands of
the
moon and Mars is a completely independent test of the inner solar
system
cataclysm and confirms our cosmochemical evidence of an asteroid
source,"
Kring, a co-author of the Science paper, said.

Kring was part of a team that earlier used an argon-argon dating
technique
in analyzing impact melt ages of lunar meteorites -- rocks ejected at
random
from the moon's surface and that landed on Earth after a million or so
years
in space. They found from the ages of the "clasts," or melted rock
fragments, in the breccia meteorites that all of the moon was bombarded
3.9
billion years ago, a true global lunar cataclysm. The Apollo lunar
sample
analysis said that asteroids account for at least 80 percent of lunar
impacts.

Comets have played a relatively minor role in inner solar system
impacts,
Strom, Malhotra and Kring also conclude from their work. Contrary to
popular
belief, probably no more than 10 percent of Earth's water has come from
comets, Strom said.

After the Late Heavy Bombardment, terrestrial surfaces were so
completely
altered that no surface older than 3.9 billion years can be dated using
the
cratering record. Older rocks and minerals are found on the moon and
Earth,
but they are fragments of older surfaces that were broken up by
impacts, the
researchers said.

Strom said that if Earth had oceans between 4.4 billion and 4 billion
years
ago, as other geological evidence suggests, those oceans must have been
vaporized by the asteroid impacts during the cataclysm.

Kring also has developed a hypothesis that suggests that the impact
events
during Late Heavy Bombardment generated vast subsurface hydrothermal
systems
that were critical to the early development of life. He estimated that
the
inner solar system cataclysm produced more than 20,000 craters between
10
kilometers to 1,000 kilometers in diameter on Earth.

Inner solar system cratering dynamics changed dramatically after the
Late
Heavy Bombardment. From then on, the impact cratering record reflects
that
most objects hitting inner solar system surfaces have been near-Earth
asteroids, smaller asteroids from the main belt that are nudged into
terrestrial-crossing orbits by a size-selective phenomenon called the
Yarkovsky Effect.

The effect has to do with the way asteroids unevenly absorb and
re-radiate
the sun's energy. Over tens of millions of years, the effect is large
enough
to push asteroids smaller than 20 kilometers across into the jovian
resonances, or gaps, that deliver them to terrestrial-crossing orbits.
The
smaller the asteroid, the more it is influenced by the Yarkovsky
Effect.

Planetary geologists have tried counting craters and their size
distribution to get absolute ages for surfaces on the planets and
moons.

"But until we knew the origin of the projectiles, there has been so
much
uncertainty that I thought it could lead to enormous error," Strom
said.
"And now I know I'm right. For example, people have based the geologic
history of Mars on the heavy bombardment cratering record, and it's
wrong
because they're using only one cratering curve, not two."

Attempts to date outer solar system bodies using the inner solar system
cratering record is completely wrong, Strom said. But it should be
possible
to more accurately date inner solar system surfaces once researchers
determine the cratering rate from the near-Earth asteroid bombardment,
he
added.

The authors of the Science paper are Strom, Malhotra and Kring from the
University of Arizona Lunar and Planetary Laboratory, and Takashi Ito
and
Fumi Yoshida of National Astronomical Observatory, Tokyo, Japan.

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Contact Information

Robert Strom
520-621-2720 (office) 520-299-3742 (home)

Renu Malhotra
520-626-5899 (office)
617-496-8380 or 520-241-8367

David Kring
520-621-2024 (office)
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